Chapter 13 – Heat Treatment of Steels

Heat Treating – defined as the controlled heating and cooling of metals for the primary purpose of altering their properties (strength, ductility, hardness, toughness, machinability, etc)

Can be done for Strengthening Purposes (converting structure to martensite)

Can be done for Softening and Conditioning Purposes (annealing, tempering, etc.)

First, a basic review of metallurgy!

1.5 The Nature of Metals:

Characterized by:

1. Valence electrons of 1,2 or 3 – see periodic table

2. Primary bonding between electrons called metallic bonding:

Valence electrons not “bonded” to particular atom but shared and free to drift through the entire metal

3. Properties include: good conductors of electricity and heat, not transparent, quite strong yet deformable!

Crystalline structures (i.e. metals) atoms are arranged in unit cells – 4 common cells shown above

How do Metal Crystals Fail?? Answer: Slip due to dislocations

Theoretical Strength of Metal

• Strength, Su should be approximately E/10 if based on atomic bond.

• E/10 = 3,000 ksi for steel >>> actual Su which is between approximately 30 ksi to 200 ksi

• Why?????

• DEFECTS!!!

Types of Defects:

• Surface Defects– Grain boundaries

• Point Defects– Vacancy, substitutional (atom replaces host),

interstitial (atom squeezes in between host), impurity

• Line Defects – Edge dislocations, screw dislocations

= good defect!

Defects in crystals. (a) Vacancies–missing atoms. (b) Foreign (solute) atom on interstitial and substitutional sites.

(c) Line Defect = A dislocation–an extra half-plane of atoms. (d) Grain boundaries.

Alloying and heat treating

Little impact on strength

Course GB = weak, Fine GB = strong and ductile

Greatest impact on strength and ductility!!

What is the most significant defect?Answer: The line defect (edge dislocation or screw dislocation)

(a) Making a dislocation by cutting, slipping and rejoining bonds across a slip plane.

(b) The atom configuration at an edge dislocation in a simple cubic crystal. The configurations in other

crystal structures are more complex but the principle remains the same.

Line Defects – How metals fail:

Slip due to line defects (aka dislocations)

An initially perfect crystal is shown in (a). The passage of the dislocation across the slip plan, shown in

the sequence (b), (c) and (d), shears the upper part of the crystal over the lower part

by the slip vector b. When it leaves the crystal has suffered a shear strain .

Slip due to line defects (aka dislocations)

A screw dislocation. The slip vector b is parallel to the dislocation line S—S.

Dislocation motion causes extension

Millions of dislocations produce the noticeable “yield marks” seen below in a simple tensile specimen:

How to Strengthen Metals:• Key: prevent dislocations from moving through crystal Key: prevent dislocations from moving through crystal

structure!!!structure!!!• Finer grain boundries – can be done by recrystallizing (and

cold working)• Increase dislocation density via COLD WORKING (strain

hardening)• Add alloying elements to give –SOLID SOLUTION

HARDENING.• Add alloying elements to give precipitates or dispersed

particles – PRECIPITATION HARDENING (aka Heat Treat)• DISPERSION HARDENING– fine particles (carbon) impede

dislocation movement.– Referred to as Quench Hardening, Austenitizing and Quench or

simply “Heat Treat”.– Generally 3 steps: heat to austenite T, rapid quench, then temper.

Several cells form a crystal, if many crystals are growing in a melt at the same time, where they meet = grain boundry as shown below:

d

k yoy

Mat’l constants

Average grain diameter

Called Hall-Petch equation

The Effect of Grain Boundries:

• Dislocations pile up at GB and can’t go further – this effectively strengthens the crystal!

Work Hardening

Strain hardening index

Work hardening, or strain hardening, results in an increase in the strength of a material due to plastic deformation.

Plastic deformation = adding dislocations – as dislocation density increases, they tend to “tie up” and don’t move.

Ludwik’s Equation:

Hot finishing = 2 benefitCold finishing = 1 benefits

Solid Solution Strengthening (AKA Alloying):

= strengthening by deliberate additions of impurities (alloying elements) which act as barriers to dislocation movement.

Example: addition of zinc to copper making the alloy brass (copper dissolves up to 30% zinc). Zinc atoms replace copper atoms to form random substitutional solid solution. The zinc atoms are bigger than copper and by squeezing into the copper lattice, they distort it making it harder for dislocations to move.

Zinc added to copper = brass. Zinc atoms are bigger and therefore distort lattice!

Cr and Ni to Fe, etc…

Dispersion and Precipitate Strengthening (aka Heat Treat):

Disperse small strong particles (i.e. carbon) to impede dislocations

Successive positions of a dislocation as it bypasses particles that obstruct its motion.

The critical configuration is that with the tightest curvature, shown in (b).

Dispersion and Precipitate Strengthening (aka Heat Treat):

This is dispersion and precipitate strengthening

This is solution hardening (alloying)

How to strengthen metals:

Other strengthening methods include remelt to remove impurities, hot roll to reduce grain size

Strengthening mechanisms and the consequent drop in ductility, here shown for copper alloys.

The mechanisms are frequently combined. The greater the strength,

the lower the ductility (the elongation to fracture, εf).

Add zinc to make brass

Watch 6 min tape!

Now the Fun Stuff:

• HEAT TREATMENT OF STEELS:

Steel Crystal Structures:•Ferrite – BCC iron w/ carbon in solid solution (soft, ductile, magnetic)

•Austenite – FCC iron with carbon in solid solution (soft, moderate strength, non-magnetic)

•Cementite – Compound of carbon and iron FE3C (Hard and brittle)

•Pearlite – alternate layers of ferrite and cementite.

•Martensite – iron – carbon w/ body centered tetragonal – result of heat treat and quenchHT: ferrite then austentite then martensite

Heat Treatment of Steels for Strength:

• Steel = 0.06% to 1.0% carbon• Must have a carbon content of at least .6% (ideally) to

heat treat.• Must heat to austenitic temperature range.• Must rapid quench to prevent formation of equilibrium

products.• Basically crystal structure changes from BCC to FCC at

high Temp.• The FCC can hold more carbon in solution and on rapid

cooling the crystal structure wants to return to its BCC structure. It cannot due to trapped carbon atoms. The net result is a distorted crystal structure called body centered tetragonal called martensite.

• Almost always followed by tempering.

Final step: Temper!

Heat treating for strength!

10.4 Direct Hardening – Austenitizing and quench:

• Austenitizing – again taking a steel with .6% carbon or greater and heating to the austenite region.

• Rapid quench to trap the carbon in the crystal structure – called martensite (BCT)

• Quench requirements determined from isothermal transformation diagram (IT diagram).

• Get “Through” Hardness!!!

Heat to austenite range. Want to be close to transformation temperature to get fine grain structure.

Austenitizing:

For this particular steel want to cool from about 1400 F to <400 F in about 1 second!

Quenching:

• Depending on how fast steel must be quenched (from IT diagram), the heat treater will determine type of quenching required:– Water (most severe)– Oil– Molten Salt– Gas/ Air (least severe)– Many phases in between!!! Ex: add water/polymer to

water reduces quench time! Adding 10% sodium hydroxide or salt will have twice the cooling rate!

• Same requirements as austenitizing:– Must have sufficient carbon levels (>0.4%)– Heat to austenite region and quench

• Why do?– When only desire a select region to be hardened:

Knives, gears, etc.– Object to big to heat in furnace! Large casting w/

wear surface

• Types:– Flame hardening, induction hardening, laser beam

hardening

13.4 Direct Hardening - Selective Hardening :

Flame Hardening:

Induction Hardening

Diffusion Hardening (aka Case Hardening):

• Why do?– Carbon content to low to through harden with

previous processes.– Desire hardness only in select area– More controlled versus flame hardening and induction

hardening.– Can get VERY hard local areas (i.e. HRC of 60 or

greater)– Interstitial diffusion when tiny solute atoms diffuce into

spaces of host atoms– Substitiutional diffusion when diffusion atoms to big to

occupy interstitial sites – then must occupy vacancies

Diffusion Hardening:

• Requirements:– High temp (> 900 F)– Host metal must have low concentration of

the diffusing species– Must be atomic suitability between diffusing

species and host metal

Diffusion Hardening:

• Most Common Types:– Carburizing– Nitriding– Carbonitriding– Cyaniding

Diffusion Hardening - Carburizing:

• Pack carburizing most common:– Part surrounded by charcoal treated with

activating chemical – then heated to austenite temperature.

– Charcoal forms CO2 gas which reacts with excess carbon in charcoal to form CO.

– CO reacts with low-carbon steel surface to form atomic carbon

– The atomic carbon diffuses into the surface– Must then be quenched to get hardness!

Diffusion Hardening - Nitriding:

• Nitrogen diffused into surface being treated. Nitrogen reacts with steel to form very hard iron and alloy nitrogen compounds.

• Process does not require quenching – big advantage.

• The case can include a white layer which can be brittle – disadvantage

• More expensive than carburizing

Reduction process: 2NH3 2N + 3H2

Source of nitrogen

13.6 Softening and Conditioning -

• Recrystallization

• Annealing– Process anneal– Stress relief anneal– Normalizing

• Tempering

13.6 Softening and Conditioning - Recrystallization

• Done often with cold working processes

• Limit to how much steel can be cold worked before it becomes too brittle.

• This process heats steel up so grains return to their original size prior to subsequent cold working processes.

• Also done to refine coarse grains

13.6 Softening and Conditioning - Annealing

• Annealing – primary purpose is to soften the steel and prepare it for additional processing such as cold forming or machining.

• If already cold worked - allows recrystallization.

13.6 Softening and Conditioning - Annealing

• What does it do?1. Reduce hardness

2. Remove residual stress (stress relief)

3. Improve toughness

4. Restore ductility

5. Refine grain size

13.6 Softening and Conditioning - Annealing

• Process Steps:1. Heat material into the asutenite region (i.e.

above 1600F) – rule of thumb: hold steel for one hour for each one inch of thickness

2. Slowly furnace cool the steel – DO NOT QUENCH

3. Key slow cooling allows the C to precipitate out so resulting structure is coarse pearlite with excess ferrite

4. After annealing steel is quite soft and ductile

Annealing versus Austenitizing:

• End result: One softens and the other hardens!• Both involve heating steel to austenite region.• Only difference is cooling time:

– If fast (quenched) C is looked into the structure = martensite (BCT) = HARD

– If slow C precipates out leading to coarse pearlite (with excess cementite of ferrite) = SOFT

13.6 Softening and Conditioning – Other forms of Annealing

• Normalizing – use when max softness not required and cost savings desired (faster than anneal). Air cooled vs. furnace cooled.

• Process Anneal – not heated as high as full anneal.

• Stress Relief Anneal – lower temp (1,000F), slow cooled. Large castings, weldments

13.6 Softening and Conditioning - Temper

• Almost always done following heat treat as part of the austenitizing process!

• Because of lack of adequate toughness and ductility after heat treat, high carbon martensite is not a useful material despite its great strength (too brittle).

• Tempering imparts a desired amount of toughness and ductility (at the expense of strength)

13.3 Softening and Conditioning - Temper

• Typical HT steps (Summarized Again):– Austenize: Heat into stable single phase region and

HOLD for uniform chemistry single phase austenite.– Quench: Rapid cool – crystal changes from

Austenite FCC to Martensite BCT which is hard but brittle.

– Temper: A controlled reheat (BELOW AUSTENITE REGION). The material moves toward the formation of a stable two phase structure – tougher but weaker.

– Quench: The properties are then frozen in by dropping temperature to stop further diffusion

The Heat Treat Processes

13.9 Selection and Process Specification

• You should read this section on your own and know how to call out typical HT processes on your drawings!